In a short letter filed with the FCC Tuesday morning, Jennifer Rockoff, attorney advisor for the DOJ’s National Security Division, asked the FCC to defer any action on the deal:

“DOJ, [Department of Homeland Security], and FBI (“the Agencies”) are currently reviewing this matter for any national security, law enforcement, and public safety issues but have not yet completed that effort. We therefore request that the Commission defer action until such time as the Agencies notify the Commission of the completion of their review and, based on the results of such review, request appropriate action by the Commission.

“DOJ, FBI, and DHS will advise the Commission promptly upon completion of our review.”

Update 1: Though the DOJ didn’t go into detail, its review most likely centers on foreign ownership. Softbank would take a 70 percent stake in the country’s third largest mobile carrier, which would trigger automatically trigger a review from federal agencies. It’s important to note that the Justice Department’s antitrust division, which would review from a competitive standpoint, wasn’t even named in the letter.

We reached out to the FCC for a response, but commission spokesman Justin Cole told us the FCC doesn’t comment on pending deals.

This story is developing and we will update it as we get more information.